1. Field of the Invention
The present invention relates in general to optical disk pickup systems and in particular to optical disk pickup systems using current division signal transmission and methods and optical disk systems using the same.
2. Description of the Related Art
Optical disks have been used for many years for the mass storage of digital data. Some well known examples of optical disks include digital audio compact disks (CD-DAs), compact disk read-only memories (CD-ROMs) and digital video disks (DVD-RAMs, xe2x88x92ROM,+RW, xe2x88x92RW, CD-R, CD-RWs). Essentially, digital data is stored on a plastic disk with a reflective surface as a series of pits and land in the reflective surface. During playback, a beam of light is directed to the rotating reflective surface and the intensity of the photons reflected from the pits and land measured. A modulated electrical signal is generated that can be processed and the data stored on the disk recovered.
A basic configuration for the read (playback) mechanism has developed over a number of years. This configuration includes a pickup or sled which is movable so that a laser, a lens, and array of photodiodes can be positioned directly over the data being read off of the disk. As the disk turns, the photons from the laser are reflected off the pits and land and received by the photodiodes which generate electrical signals having a current that is proportional to photon density.
The multiple signals output from the photodiodes represent both data detection and servo alignment information. The summation of the high speed data channel signal, which may be composed of the signals A+B+C+D from an astigmatic photodiode array, results in a composite signal with relevant information between approximately 10 KHz and 60 MHz for current DVD players. Servo information contained in these signals however, is at frequencies less than 1 MHz down to dc (for current spindle rotation rates of  less than 6000 RPM). Because of these information rates, the data channel signal is sometimes AC-coupled to the data detection and summation circuitry mounted on an accompanying stationary circuit board. Otherwise, some degradation of the dynamic range must be accepted due to the dc content of the incoming signal.
The typical current signal generated by a photodiode is on the order of 1 uA. Transferring this signal directly down a flexible cable to the stationary circuit board therefore would seriously degrade the signal to noise ratio. Hence, transimpedance amplifiers, which convert the current from the photodiode array into a voltage for driving the cable, are mounted in the pickup to minimize noise and interference effects. The data detection, error correction, and servo systems are kept off of the pickup not only because of interference reasons, but primarily to reduce the physical size and mass of the sled. These systems are mostly digital, and switching noise on the pickup may degrade the signal to noise ratio.
One of the primary concerns about transferring data across the flexible cable as a voltage is maintaining a good signal to noise ratio. A good signal to noise ratio can be achieved by insuring that the output of the pickup electronics are driven across the flexible cable using a sufficiently high supply voltage. Notwithstanding, it would be desirable to be able to reduce the supply voltage to save power; however, to do so would reduce the amplitude of the signals being transmitted across the cable and hence reduce the signal to noise ratio. Moreover, a reduced voltage supply rail decreases signal dynamic range. What is needed therefore are methods and circuitry which maintain the signal to noise ratio for signals being transmitted across the flexible cable, even if the supply voltage is reduced.
An optical disk pickup system is disclosed using current mode signal transmission. An operational amplifier has an input for receiving an electrical signal and a feedback loop including a current path of a first transistor of a first size, the first transistor having a control terminal at a preselected voltage. A conductor is coupled to an output of the operational amplifier for transmitting the electrical signal as a current. A second transistor of a second size has a current path in series with the conductor and a control terminal coupled to the preselected voltage, the first and second transistors forming a current divider.
The use of current mode data exchanges has substantial advantages over the traditional voltage mode transmission schemes. Among other things, by using current, the supply voltage used to transmit and receive signals can be substantially reduced without a significant degradation in signal to noise ratio. Moreover, when signals are summed for transmission, current mode transmission ensures that the available voltage headroom is used efficiently.